1. Technical Field
This invention relates generally to an adaptive loading circuit and, more particularly, to an adaptive loading circuit for adaptively attenuating a differential voltage produced by a magnetic or variable reluctance sensor in response to rotation of a wheel.
2. Discussion
Inductive magnetic sensors are commonly employed for automotive applications and the like to provide timing signals which enable the determination of position and speed of a rotating wheel. For example, specific applications may include the determination of engine crankshaft position and speed (i.e., RPM) or the determination of wheel speed for anti-lock braking systems. Inductive magnetic sensors used for these types of applications are commonly referred to as variable reluctance sensors.
The variable reluctance sensor is generally located adjacent to a rotating wheel which typically has a plurality of circumferentially spaced slots formed therein. The sensor has an inductive magnetic pick-up that is generally made up of a pick-up coil wound on a permanent magnetic core. As the wheel rotates relative to the pick-up coil, an alternating voltage is generated in the pick-up coil when the slots on the wheel travel past the sensor. The alternating voltage must then be correctly decoded to recognize high or positive voltage levels. The frequency of the alternating voltage is then determined to achieve rotational speed information about the wheel.
The alternating voltage that is produced with the variable reluctance sensor has peak voltages that generally vary in amplitude according to the rotational speed of the wheel. In a number of automotive applications, the amplitude of the peak voltage may vary from approximately 250 millivolts (mV) at low end speeds to over 160 volts (V) at higher rotating speeds. In fact, some commonly employed sensors are often rated to generate up to 200 volts. However, the sensor output is usually fed to a processing module or other control device that is designed to operate within a more limited voltage range. For instance, automotive processing modules are commonly designed with 5 volt CMOS transistors in order to accommodate size and power constraints. For a 5 volt processing module, the input signal may not exceed 5 volts in order to protect the circuitry. Therefore, in order to accommodate a 0-5 Volt range, the sensor output voltage must be properly attenuated when necessary.
In the past, one problem that has remained with some approaches has involved the inability to achieve a limited input voltage without sacrificing noise immunity and accuracy. For example, one approach suggests clipping the sensor output voltage at the rails (i.e., ground and 5 volts) and then using the clipped voltage to obtain the frequency information. However, this approach is very susceptible to noise interference, especially at higher speeds where noise may account for several volts in amplitude.
One approach for attenuating the output voltage of a variable reluctance sensor is discussed in U.S. Pat. No. 5,144,233 issued to Christenson et al and entitled "Crankshaft Angular Position Voltage Developing Apparatus having Adaptive Control and Diode Control". This approach uses a resistive divider network which is controlled by the forward voltage of a diode. The above-referenced approach has the variable reluctance sensor connected in a single-ended configuration with one end of the pick-up coil connected to ground. In addition, the circuit is capable of swinging to the level of the battery. This single-ended approach operates such that when the input voltage becomes high enough in amplitude to forward bias the diode, a resistive path is established to create a resistor divider network that attenuates the input voltage. However, this approach does not teach the attenuation of a differential voltage, and may not provide sufficient attenuation control that is necessary to protect the 5 volt gates associated with the processing module.
Another approach uses an RC network to attenuate the input voltage. While conventional RC filtering approaches have provided suitable attenuation for low voltage attenuation, high voltage attenuation applications such as those associated with Direct Ignition Systems (DIS) have exhibited limited attenuation capability. That is, it is generally difficult to provide substantial voltage attenuation for high voltages and also more difficult to control the attenuation. In addition, the RC filter introduces the propensity of a phase shift between the inputs for a differential voltage, thereby potentially causing significant errors in zero-crossing position determination.
It is therefore one object of the present invention to provide for an adaptive loading circuit which is capable of handling a differential voltage that is generated by a variable reluctance sensor.
It is another object of the present invention to provide for an adaptive threshold circuit that adaptively attenuates a differential voltage developed by a magnetic sensor when the differential voltage exceeds selected threshold voltages.
It is a further object of the present invention to provide for such an adaptive loading circuit that attenuates a differential voltage to within a limited voltage range of about 0-5 volts, while maintaining noise immunity, accuracy and extending the dynamic range of the system.